The present application is a U.S. non-provisional application based upon and claiming priority from Russian Application No. 99124383 which is hereby incorporated by reference.
The invention provides a method for removing volatile organic chemicals from waste gas streams produced as a byproduct in the manufacture of phenol. Specifically, the invention provides a method for removing methanol and cumene from a waste gas stream while avoiding catalyst deactivation by phenol tar formation.
If waste gases contain small quantities of organic impurities, strictly isothermal operating conditions are used in removing the impurities. Implementation of such methods involves adding significant quantities of heat to the reaction mass. Various catalytic systems are used as industrial catalysts for removing organic impurities from waste gases. Some contain the oxides of base metals, for example chromium, cobalt, zirconium (Russian patent No. 2050976), magnesium (U.S. Pat. No. 4,673,558), manganese, copper (Russian Patent Application No. 95102026). Other catalysts contain noble metals, for example platinum (U.S. Pat. No. 5,702,836), palladium, and silver (U.S. Pat. No. 4,673,556). Recently, catalytic systems comprising noble metals on complex oxide carriers have become widespread (U.S. Pat. No. 5,585,083), in particular group VIII metals on titanium and cerium oxides (U.S. Pat. No. 4,716,859), or, for example, palladium and rhodium on cerium oxides (U.S. Pat. No. 4,919,903).
The most universal method of cleaning waste gases of C3 hydrocarbons and oxygen-containing compounds (including inorganic ones) is disclosed in U.S. Pat. No. 5,292,991, which describes a process carried out at a temperature of 600xc2x0 C. in the presence of a catalyst comprising platinum and palladium jointly deposited onto zeolite ZSM type modified by zirconium and titanium oxides. This process achieves a conversion of hydrocarbon of 98.1%. The disadvantage of this method is the necessity of heating streams of steam and gas, which, as a rule, are rich in nitrogen and steam, to a high temperature. Also, this method is only effective for cleaning the streams of light hydrocarbons which do not cause resinification of the catalyst.
Catalytic compositions containing noble metals have been developed to treat specific organic compounds. For example, a known low-temperature method according to U.S. Pat. No. 5,653,949, for cleaning waste gases of organobromine compounds uses a catalyst containing metals of the platinum group on oxides of zirconium, manganese, cerium, and cobalt recommends using a palladium/rhodium catalyst on cerium oxide to reduce the temperature to which the gas stream is heated when it is cleaned of methanol and formaldehyde. This method makes it possible to reduce the reaction temperature to 150xc2x0 C. The disadvantage of this method is that it can only remove methanol and formaldehyde from waste gases. Therefore, if the gas stream contains aromatic hydroperoxides, which cause resinification of the catalyst, this method is not effective.
U.S. Pat. No. 5,009,872, describes a method for cleaning industrial waste gases which contain aldehydes, alcohols, and ketones with up to 5 carbon atoms at a low temperature (not greater than 150xc2x0 C.) using a catalytic system comprising up to 22% of at least two noble metals supported on a special hydrophobic carrier. This method cleans the waste stream of 90% of its ethanol and 93% of its formaldehyde. The disadvantage of this method is that again, it is only possible to clean waste gases of small organic compounds, whose presence does not cause the resinification of the catalyst (e.g., methanol, ethanol, acetone, and formaldehyde).
The method of cleaning waste gases of oxygen-containing organic compounds which is closest to the proposed method involves passing waste gases over a catalyst which is 0.03-3.0% palladium, deposited onto a carrier which includes aluminum oxide. This method is described in U.S. Pat. No. 4,450,244. According to U.S. Pat. No. 4,450,244, the process is carried out in the presence of oxygen at an elevated temperature (i.e., 350 or 400xc2x0 C.), which ensures the cleaning of various organic oxygen-containing compounds to the level of 88-96% at 350xc2x0 C., and to the level of 98-100% at 400xc2x0 C. The advantage of this method is the lower content of noble metals compared with catalysts described in some of the preceding patents and the higher degree of cleaning of organic oxygen-containing compounds which it provides. The disadvantages of this method are: 1) the necessity of heating the gas stream to a high temperature, and 2) the inability to clean the gas stream of the indicated compounds in the presence of organic hydroperoxides as a consequence of resinification of the catalyst.
In view of the above, the task was to develop an efficient method for cleaning industrial waste gases, including oxygen-containing compounds, among which are aromatic hydroperoxides.
The guard bed described herein was used earlier for thermal decomposition of 1) side products which form during the synthesis of 4,4-dimethyl-1,3-dioxane (DMD), and of 2) the pyran fraction obtained in the decomposition of DMD in the process of synthesizing isoprene from formaldehyde and isobutylene (Russian Patent 1695631, and Romanian Patent 88186, of May 14, 1983). In these patents, the products undergoing thermal decomposition consist of compounds of the pyran and dioxane type, whose properties differ greatly from those of the aromatic hydroperoxides found in waste gases.
In order to simplify the technology of the process by reducing the cleaning temperature of the waste gases, including aromatic hydroperoxides, cumene, and also formic acid, it is proposed to carry out the process as described below.
The waste gas described herein comprises an aromatic hydroperoxide impurity (e.g., cumene hydroperoxide). A typical waste gas comprises cumene, cumene hydroperoxide, methanol and formic acid.
To remove impurities, the waste gas is first passed through a guard bed and second passed through a palladium catalyst. The guard bed protects the palladium catalyst be decomposing organic impurities which could lead to inactivation of the palladium catalyst due to tar formation. This makes the process feasible because the palladium catalyst is expensive and can not be replaced often in an economical process.
The guard bed has a surface area of 0.2-1.0 m2gxe2x88x921 and is roasted at a temperature of 800-1,350xc2x0 C. before use. After roasting, the guard bed contains oxides of aluminum, iron (II), magnesium, calcium, potassium, sodium, titanium (IV), and silicon, which have the following mass ratios (%):
The palladium catalyst contains 0.1 to 3% palladium supported on active aluminum oxide. A preferred catalyst of this type is Sud Chemie T-2864F catalyst comprising 0.7 wt. % palladium on alumin on 1/20xe2x80x3 extrudates, in pre-reduced state, at a bulk density of 35 lb./ft3.
In a preferred embodiment of the invention, the weight ratio of the guard bed to a palladium bed catalysts is from 0.5:1 to 2:1.
The process is preferably conducted maintaining the temperature at 120 to 160xc2x0 C. at both catalysts.
In operation, the waste gas is first passed through the guard bed with oxygen present. Next, the waste gas is passed through the palladium catalyst. In the case of waste gases containing formic acid, the guard bed is though to remove the formic acid, thereby avoiding formation of tar on the palladium bed. The palladium bed if effective to remove cumene hydroperoxide.
The proposed method makes it possible to clean industrial waste gases of harmful substances (for example, methanol, cumene), with the degree of their destruction not falling below 97-98%, in the presence of compounds (for example, aromatic hydroperoxides), which cause resin formation on other catalytic compositions.